1. Field of the Invention
This invention relates generally to echo cancellers and, more particularly, to echo cancellers and echo cancelling methods for reducing cross coupling echoes in communication systems.
2. Description of the Related Art
Echo cancellers used in communication systems to reduce echoes caused by cross coupling of an incoming received signal received at a communication unit with an outgoing analog output signal are well known. Referring to FIG. 1, a PRIOR ART communication system 10 is seen to have a plurality of communication units, such as communication units 12 and 14, which are interconnected via a connection medium 16, such as a telephonic switching system or airwave transceiver.
Associated with each of the communication units, such as shown with respect to communication unit 14, is a digital echo canceller 18 contained within a common housing 20 together with the associated communication unit 14.
An incoming signal from output 22 of communication unit 12 passes through the connection medium 16 and an input 17 and output 19 of digital echo canceller 18 and is received at an input terminal 24 of communication unit 14. The incoming received signal at input terminal 24 is cross coupled via a cross coupling path 26 and creates a cross coupling echo component 28 combined with an originating signal 30 originating from or passing through the communication unit 14 to form a composite analog output signal 34 produced on output terminal 32. The composite signal 34 produced on output terminal 32, composed of the originating analog output signal 30 and the analog cross coupling echo component 28, is applied to an input terminal 36 of the digital echo canceller 18. The digital echo canceller converts the analog signal 34 to digital form and reduces the cross coupling echo component 28 before passing the echo reduced composite signal to its output 38.
Referring to FIG. 2, in the case of the communication unit 14 being a telephonic unit 40 (e.g. a regular telephone, a telephonic handset, a speaker telephone or stand alone speaker-microphone set, a cordless telephone or a cellular telephone or full duplex two-way radio) with a speaker 42 and a microphone 44, the cross coupling path 26 is an acoustic path for sound waves passing from the speaker 42 to the microphone 44. The echo component 28 results from the sound from the speaker 42 being picked up by the microphone 44. The originating signal is obtained from the local sounds, such as a person's voice, which is picked up by the microphone 44 and converted to the originating analog output signal 30. In such case, the digital echo canceller 18 is incorporated into and forms part of a digital signal processor 46, such as a DSP 1604 digital signal processor made by the Microelectronics Division of Lucent Technologies Inc. The composite analog output signal 34 is converted to digital form by an analog-to-digital, or A/D, converter 48 and the echo reduced digital signal on output 38 of the digital echo canceller 18 is then reconverted to analog form by a D/A converter 50. The D/A converters are "codecs", model number T7525.
Likewise, the analog signal at input 17 coming from the communication unit 12 is converted to digital form by an A/D converter 52 before being applied to an input 54 of the digital echo canceller 18. The received signal from A/D converter 52 is then reconverted to analog form by a D/A converter 56 to provide the analog received signal to input 24 of communication unit 14, in this case an input 58 of speaker 42 of telephone 40. The digital echo canceller 18 functions to combine the digital sample of the received signal at input 54 with the digital form of the composite analog output signal 34 appearing at the output of A/D converter 48 to cancel the corresponding digital form of the analog echo component 28 which has also been converted to digital form by A/D converter 48.
Referring now to FIG. 3, in the case of the communication unit 14 being a telephonic hybrid interface circuit, or hybrid, 60 and the communication unit 12 being a modem 62, the same digital signal processor 46, with digital echo canceller 18, D/A converter 56 and A/D converter 48 are employed. However, since the modem 62 is equipped both to transmit digital signals on its output 64 connected to input terminal 17 and to receive digital signals at its input 66 connected with terminal 38, the A/D converter 52 and the D/A converter 50 are eliminated.
The modem 62 also has an input terminal 65 and an output terminal 67 connected with a computer, facsimile machine or other like digital data communication equipment. The input 24 of the communication unit 14, in this case hybrid 60, is a two wire line carrying the incoming received signal from modem 62 in analog form, and the output terminal 32 is a two wire line carrying the composite analog output signal 34. The composite signal 34 is composed of the cross coupling echo component received via the cross coupling path 26 from the input 24 and the originating audio modulated data signal 30 obtained from a hybrid line terminal 64 which, in turn, is connected via a telephonic switching system or the like to a remote modem (not shown).
These known digital echo cancellers require highly accurate analog-to-digital and digital-to-analog conversion to achieve a high degree of echo cancellation. An example of an echo canceller using only digital processing is shown in U.S. Pat. No. 5,598,468 issued Jan. 28, 1997 to Ammicht et al. for "Method and Apparatus for Echo Removal in a Communication System". At high speeds above 50-KBPS, signal converters are expensive as also are the high speed processors needed for digital echo cancellation. Accordingly, it is also known to provide high speed echo cancellation using only analog technology without the use of digital-to-analog or analog-to-digital conversion. An example of such an analog system is shown in U.S. Pat. No. 4,811,342 issued Mar. 7, 1989 to Huang for "High Speed Analog Echo Canceller".
While these prior digital echo cancellers and prior analog echo cancellers function to achieve their purpose in reducing echoes, complete echo removal is not achieved.